1. Technical Field
This invention relates generally to programmable solid-state devices, and more particularly, to permanently programmable solid-state devices.
2. Related Art
Solid-state imaging devices (also referred to as image devices or imagers) have broad applications in many areas including commercial, consumer, industrial, medical, defense and scientific fields. Solid-state imaging devices convert a received image from an object into a signal indicative of the received image. Solid-state imaging devices are fabricated from semiconductor materials (such as silicon or gallium arsenide) and include photosensitive imaging arrays (photosensors) of light detecting picture elements, or pixels, (also known as photodetectors) interconnected to generate analog signals representative of the received image. Examples of solid-state imaging devices include charge coupled devices (CCD), photodiode arrays, charge injection devices (CID), hybrid focal plane arrays and complementary metal oxide semiconductor (CMOS) imaging devices.
Photosensors of the solid-state imaging devices are typically formed in an array structure, with rows and columns of photodetectors (such as photodiodes, photoconductors, photocapacitors or photogates) which generate photo-charges proportional to the radiation (such as light) reflected from an object and received by the photosensor. The period of exposure of a photosensor by incident radiation is referred to generally as the integration period. An exposure shutter may control exposure of the photosensor to incident photons. The exposure shutter may be, for example, electrically, mechanically or electro-magnetically operated. The photo-charges are created by photons striking the surface of the solid-state (i.e. semiconductor) material of the photodetectors within the photosensor. As photons strike a photodetector, free charge carriers (i.e., electron-hole pairs) are generated in an amount proportional to the incident photon radiation. The signals from each photodetector may be utilized, for example, to display a corresponding image on a monitor or to provide information about the optical image.
Each photodetector includes a detecting area (also known as the photosensitive area or the detector area) and photodetector circuitry within a common integrated circuit die. The photodetectors receive a portion of the reflected light received at the solid-state imaging device, and collect photo-charges corresponding to the incident radiation intensity falling upon the photodetector' detecting area of the die. The photo-charges collected by each photodetector are converted to an output analog signal (analog charge signal) or a potential representative of the level of energy reflected from a respective portion of the object. The analog signal (or potential) is then converted to a digital voltage value and processed to create an image.
The detecting area of each photodetector is typically smaller than the actual physical photodetector dimensions because of the space constraints caused by manufacturing processes, presence of the photodetector circuitry (such as the active elements in CMOS photodetectors) in addition to the photodetector and the proximity of adjacent photodetectors. The percentage ratio of the detector area to the overall photodetector (i.e., pixel) area is typically referred to as the optical “fill factor.”
Where CCD imaging devices typically suffer from low yields and high power consumption and are often expensive to produce and require a specialized fabrication facility dedicated to CCDs, CMOS imaging devices are lower in overall require less power, and have a higher level of circuit integration on the die that enables “camera-on-a-chip” capabilities. Additionally, CMOS imaging devices are typically manufactured in a standard CMOS wafer fabrication facility, and may also reduce the size of the solid-state imaging device through circuit integration.
Additionally, CMOS technology is capable of significantly higher access rates (i.e. frame rates) than CCD technology at the same or lower levels of circuit noise, because elements may be designed to operate in parallel. In CCD circuits, a single amplifier transforms charge to voltage and must support the total data rate of the imager frame. As a result in CCD imaging devices, the amplifier noise becomes dominant for image sizes over several hundred thousand photodetectors at about 30 frames per second (FPS). In contrast, in CMOS imaging devices, multiple amplifiers are utilized to allow a longer settling time between applications and a significant y higher frame rate while maintaining superior noise rejection. Additionally, CMOS imagers may easily be equipped with a precision analog-to-digital converter (“ADC”) on the imager chip.
CCDs devices having photosensors are commonly found in digital imaging devices such as digital cameras and video cameras. Digital imaging devices may alternatively employ CMOS technology for generating digital images. In the manufacture of conventional CCD and CMOS imaging devices, the re are often a number of defective photodetectors in each array. These defective photodetectors may produce a signal indicating the presence of bright light even when no light is incident on the photodetector, or may produce a signal indicating no light even when bright light is incident on the photodetector. Defective photodetectors (also referred to as defective pixels) are caused by a number of factors including, a local crystal defect within the semiconductor material. These defective pixels often result in degraded picture quality from the solid-state imaging device.
An image processor typically processes the digital voltage values corresponding to the photo-charges collected by the photodetectors in the photosensor. In certain circumstances, the image processor may perform image correction to minimize degradation of the image due to defective pixels in the photosensor. Typically, the positions or locations of the defective pixels are fixed within the photosensor. To facilitate image correction, the location of defective pixels may be identified and stored in advance of image processing. In conventional solid-state imaging devices, the locations of the defective pixels are typically determined during the digital camera assembly stage. In a known example, one way of identifying the defective pixels is to expose the photosensor of the solid-state imaging device with uniform incident photons (i.e., illuminate the photosensor with light) for a predetermined time period, so that all photodetectors in the photosensor receive the same amount of light during that time period. The charge collected in each pixel is then accessed.
Each pixel is expected to collect the same amount of charge because each pixel structure is identical and exposed with the same amount of incident photons for the same period of time. Thus, in this example, the defective pixels are those which did not collect the expected amount of charge. The processor then identifies and generates a list of defective pixel locations within the photosensor.
Once the defective pixel locations are known, they are recorded in an external memory chip. In a known system, the memory chips typically utilized are erasable programmable read-only memory chips, or “EPROMs.” The EPROMs are typically placed on a printed circuit board (PCB) for the device (e.g., a digital camera) in which the associated photodetector is to be utilized.
There are several disadvantages to storing defective pixels in this manner. First, utilizing an external memory chip adds extra steps and thus increases the assembly time of the device in which the memory chip is utilized (e.g., a digital camera, or a USB “universal-serial bus” peripheral). In addition, the size of the device in which the memory chip is utilized must be increased to provide space for the separate memory chip. Furthermore, the overall cost to manufacture the device is increased due to both the increased assembly time, and the requirement of an extra external memory chip on the PCB. Finally, the need for an additional memory chip increases the number of external electrical connections and increases the complexity of the device and opportunities of malfunctions.
Thus, there is a need in the art for a process and system for storing addresses of defective pixels in an imaging device for later retrieval that decreases the overall assembly time of the device in which the solid-state imaging device is utilized, is relatively inexpensive to manufacture, is reduced in size, and is relatively simple to implement in the fabrication process.